When farmers cultivate their land, they usually do so by using large and heavy machinery. They make farming more efficient and make the farmers' everyday lives much easier, but there is a downside. Heavy machinery compresses the soil, damaging its ability to absorb both water and nutrients. This affects the ability of crops to grow and, looking at the bigger picture, soil compaction significantly increases the risk of nitrous oxide emissions. 

"Nitrous oxide is a very powerful greenhouse gas, which is why we have chosen to review all the knowledge we have on how soil compaction affects nitrous oxide emissions. Up to now, there has been a lot of focus on fertilisation and the use of plant residues, i.e. the supply of nitrogen and carbon itself, but soil physical conditions are also part of the equation," explains Professor Lars J. Munkholm from the Department of Agroecology at Aarhus University. 

Up to 42 times more nitrous oxide

This is not brand-new knowledge. When soil is compressed by heavy machinery or even animals in the field, it increases nitrous oxide emissions, that is a known fact. But it's a topic that Lars J. Munkholm says hasn't been studied enough. As part of the TRACE Soils project, an EJP Soil project, he has been looking at the soil physical conditions that can affect the risk of nitrous oxide emissions. 

"We have studied how soil compaction on arable land in rotation, pastures and in forests affects nitrous oxide emissions. And we found that between 1.3 and 42 times as much nitrous oxide is emitted in areas with compacted soil, and it was particularly cultivated land and grasslands that were badly affected," he says. 

For nitrous oxide to form, a number of factors need to be present. There must be partially oxygen-free conditions, there must be available nitrogen that can be metabolised, and there must be carbon for the micro-organisms to live on. So, soil compaction plays a role because it can help create an oxygen-free environment.

"When the soil is compacted, it has smaller pore openings, and the pores are not connected; in other words, they're not continuous, and that inhibits the transport of oxygen in the soil. When this happens in areas that have both nitrogen and carbon available at the same time, nitrous oxide is produced," says Lars J. Munkholm. 

We work with a factor of two

So, soil compaction increases the risk of nitrous oxide emissions by up to 42 times. But, explains Lars J. Munkholm, 42 is an extreme, and a factor of two is used, meaning that areas with soil compaction emit twice as much nitrous oxide as areas where the soil is not compacted. 

"It's important to point out that it's especially the places in the field where you have a combination of compaction and added nitrogen in the form of fertiliser or plant residues. It is this combination that is causing the hotspots for nitrous oxide emissions," says Lars J. Munkholm. 

Just as with the whole field, the packed areas emit by far the most nitrous oxide at the times when nitrogen is applied. This is where significant peaks occur, and they will be more pronounced in, for example, the wheel tracks. In other words, the risk is twice as high in the field's wheel tracks and headland. 

"In the Danish context, it is particularly under wet conditions that you see a fluctuation, that is in the winter and spring periods. For example, bringing out slurry in the spring is problematic, as heavy machinery is used and both nitrogen and carbon are added to the field," says Lars J. Munkholm. 

But what can be done to avoid the doubling of nitrous oxide from hotspots in the field? According to the professor, the farmer has to drive more intelligently. That means driving fewer times and with the right machines, the right tyres and the right tyre pressure. 

Research continues

There is great potential to mitigate climate change if nitrous oxide emissions are reduced, so the research doesn't stop there. Several projects have been launched.

"We did this study to set the stage, so that we can continue to work on the issue in the future in some of our new projects," says Lars J. Munkholm, who is involved in two new Innomission projects funded by the Innovation Fund. Here, the researchers will look at soil physics and mechanisms in relation to biochar and fertilisation. 

"In addition, we have a project called Solgras, which is led by Mathieu Lamandé. It is a GUDP project focusing on the development of new technology to optimise the driving in the field when harvesting grass in order to increase yields and reduce the impact on climate (including nitrous oxide) and the environment," says Lars J. Munkholm, who is full of ideas for new projects within the topic.